1. Field of the Invention
This invention relates to improvements in monopolar prismatic cells and more particularly to providing an improved high temperature current collecting medium that is lightweight and corrosion resistant.
2. Description of the Prior Art
Each cell comprises a positive and a negative electrode physically separated from one another by an ionic conductor.
The positive electrode, negative electrode and separator of each cell contains an electrolyte which is liquid at the operating temperature. High temperature rechargeable batteries require a chemically inert, highly conductive current collector for the positive electrode. The positive electrode contains a metal disulfide, typically FeS.sub.2, CoS.sub.2 or NiS.sub.2 or mixtures thereof, which has high sulfur reactivity at the operating temperature of 300.degree. C. to 500.degree. C. and is especially corrosive during recharge. This corrosive environment in the cell limits the material suitable for use in the positive current collector.
If maximum performance is to be realized from an operating battery over an extended period of time, good electrical contact must be maintained at all times between the electrodes and current collection system. Thus, the current collection system should be fabricated from components that retain their physical envelope dimensions when subjected to elevated temperatures.
An additional problem that develops with lithium alloy/metal sulfide high temperature molten salt electrolyte batteries is that due to the nature of the very reactive and highly corrosive compounds they contain, the current collector components must be highly durable and corrosion resistant. The difficult problem in affecting a corrosion resistant current collector system is that the current collector components must also be electrically conductive to an acceptable degree.
Heretofore, metals have usually been considered for the current collectors because of the electrical conductivity requirement. However, alloys typically used in the industry do not meet the corrosion resistance requirement without additional treatment. Refractory metals have been found that meet the corrosion resistance requirement. These refractory metals include molybdenum and tungsten. Refractory metals are relatively costly and are relatively heavy so as to add weight to the cell stack. If some other material is to be used, it will be necessary to coat it with an electrically conductive coating which is highly resistant to attack by the electrode materials or the electrolyte. However, even tiny imperfections (pin holes) in the coating will rapidly result in corrosion of the surrounding casing. Depending on the coating material, it is even possible that the presence of the coating material will accelerate an attack of the surrounding metal in the vicinity of the pin holes, compared to the rate of attack on an uncoated plate.